Tempering bath



Nov. 18, 1941. R. K. DAY

TEMPERING BATH Filed yJune 22, 1938 225 25o rs 30o 325 BATH 71E/wp. "C

37s voor 425 45o ATH 727W. "C

A TTORNEYS.

IN V EN TOR.

BY Cafl ,t

Patented Nov. 18, 1941 TERIPERING BATH Ralph K. Day, Snyder, N. Y., assignor to Corning' Glass Works, Corning, N. Y., a corporation of New York 'Application June z2, 193s, s.eria1No.215,zs1

(ci. isi-fsm 4 Claims.

This invention relates to the tempering of glass articles and more particularly to quenching baths for use in the tempering process.

The process of tempering a glass article consists of the steps of heating the article to a temperature slightly below the ysoftening tempera ture of the glass from which it is made and rapidly cooling the article to a temperature below the annealing temperature of the glass by quenching it in a cooling bath or applying jets of gas to its surface. This process sets up conditions of permanent stress in the articler so that its surface layers are in compression and its interior is in tension.. The amount of stress which can be set up in an article without breaking it is limited primarily by the vstrength of the particular glass composition, but within this limit the strength .of the tempered article increases with the degree It has been. found that the of stress therein. faster heat is removed from a given article the greater will be the degree of stress set up.

A considerable number of substances have been t' used as quenching baths in the glass tempering process.

Most common of these are the oils, molten metals and salts and synthetie materials such as the light chlorinated resins. From the commercial standpoints oi!A cost, durability, and ease of control and manipulation the molten salts, singly and in combination, have proved most'satisfactory. However, due to the natural characteristics of these materials such as their specific heats and melting points', therate at which they will remove heat from a hot glass article l`s delinitely limited. yWith many glass compositions this limitationdoes not interfere with the produc-v tion of the maximum desired temper, but in the case of low expansion glassesl it has`been impossible to secure the maximum desired degree of stress using molten salts alone or as eutectic cbmpositions.

The object of this invention -s la quenching A tice of this inventionwhich .features the yincorporation of a small amount oi water in a bath of molten salt or salts and the continuous introduction of water in suilicient volume'to maintain rsuited in the destruction of the articles.

the percentage of water in the bath relatively constant f While water alone has been used asfa quenching `bath in the production of highly stressed' Prince Ruperts drops, it is such a, severe chilling medium that attempts to use it in the tempering of shaped commercial articles have always re- In the present instance it has been found that a small amount of water may be added to molten salts such as the nitrates, nitrites, chlbrides, and' sulphates of barium, calcium, sodium, and potass ium producing baths which have markedly superior chilling ability to the `molten salts alone. Such baths are relatively stable below 300 C. and may be used over a range of temperatures depending on the composition of the salts, of

The manner in which water is retained in molten salts at temperatures above 100 C. is not fully understoodfbut it has been determined exhour/s heating at 300 C. are required to bring the salts to equilibrium conditions at which no.

I further water is driven off. At 400 C. thistime is reducedn to some three orffour hours, while at 500 C. one hour is sufcient Accordingly, it will be seenthat for quenching baths operating at from C. to about 550" C., small percentages of water can he maintained in amolten salt bath with relative ease, only enough water being introduced, vcontinuously or from time to time, to make up the .water being driven oi at that particular operating temperature. This water may' be introduced either as a liquid or as steam, steam amount of water introduced, but reaches substan-j tially its maximum value with about, I15% of water., A second generalization which holds true isthat the percentage increase in chilling power of va bath due to the addition of a given percentage of water decreases as theoperating tempera-` ture of the bath increases.A Thus, the greatest benefits are obtained by the addition of small persented showing the results obtained in actual op-l eration under specific circumstances. Since the value of maximum tension set in a piece of glass is a. measure of the chilling power of a bath, the eiect of the addition of water to the salt baths in question is shown in values of tension produced.

In preparing the data from which the curves of Fig. 1 were plotted, slabs of borosilicatelglass of the composition B2 of the Sullivan and Taylor Patent No. 1,304,623 2" x 4" x 1A," were preheated at 450 C. heated at 900 C. forl one and one-half minutes and immediately quenched in salt baths of the desired composition, temperature and water content. The, solidcurves Aand B were obtained with a salt composition of 80% (KNOa-l-NaNOs eutectic) and Ba (N002, 5% water being added in the case of curve B. Curves C and D were obtained by quenching similar slabssimilarly heated in a salt bath composed of KNO3-I-20% NaNOs`+35% Ca(lNOs) 2 5% water being added in the case of curve D.

Fig. 2. represents the results obtained by preheating 2" x 4 x 1A" slabs of Libbey-Owens- Ford plate glass at 400 C., heating for thirty n seconds at 800 C. Yand quenching in a bath composed of 80% (KNOa-l-NaNOa eutectic) and 20% Ba(NO3) 2. Curve E indicates the results obtained with a water free salt bath while curve F indicates results with 5% of water added to the salt bath.

Fig. 3 shows the results obtained by adding increasing percentages of water to salt baths bath of curve H as KNOa-i-NaNO: `eutectic heldy at 225 C.; that of curve J was (KNOs-i-NaNOs) +Ca(NO3) held at 225 C.;`-that "of curve K was KNo3+NaNo3 +Ba No3 held at 250 C. The results with a mixture of 66.6% NaNOa and 33% Ca(NO3)z were substantially identical with those of curve J and are not plotted.

Not only has it been found that the addition i of small quantities of water materially increases the chilling power of molten salt baths, but it has also been found that-these salt baths have been made effective for use over a wider temperaturezrange. Thus, when 5%'of water is added to the eutectic mixture of KNO: and NaNOz the water apparently goes into solution in the salt lowering its melting point and providing a working range down to 105 C. some 35 C. below the normal melting point of the eutectic. In the case of the KN-l-NaNOs eutectlc this effect is not quite so pronounced but lowers the minlmum working temperature from 220 C. to 190 C. Similar effects are noted with the other substances used.

While the results obtained by the addition of small quantities of Water to various specic salts and salt baths have been set forth in detail these results have been shown by way of illustration and not of limitation and the scope o1' the invention is to be limited solely by the terms of the following claims.

I claim: 1. The method of tempering a glass article .which comprises heating the article to a temperature approaching the softening temperature of the glass from which it is made and quenching the hot article in a bath of molten salt selected from thexgroup consisting of the nitrates, nitrites, chlorides, and'sulphates of barium, calcium, sodium, and potassium maintained at a temperature above its melting temperature, and maintaining a small, regulable quantity of water in said bath during said quenching operation.

2. The method of tempering -a glass article which comprises heating the article to a temperature approaching the softening' temperature of the glass from which it is made and quenching the hot article in a liquid mixture containing Water and an inorganic salt selected from the group consisting of the nitrates, nitrites, chlorides, and sulphates of barium, calcium, sodium, and potassium maintained at a temperature of from C. to 550 C., and adding sufficient Water to said mixture during the quenching operation to maintain its'water content substantially constant.

`3. The method of tempering a glass article which comprises heating the article to a temperature approaching the softening temperature of the glass from which it is made and quenching the hot article in a liquid mixture of salts selected from the group consisting of the nitrates, nitrites, chlorides, and sulphates of barium, calcium, sodium, and potassium-containing appreciabl amounts of water and maintained at a temperature above the melting point of the salts, K

maintained at a temperature above the melting point of the salts, and maintaining said water content substantially constant during the quenching operation by the additionof makeup water.

RALPH K. DAY.- 

